The study of maturational progress in the whitespotted rabbitfish (Siganus sutor) under different photoperiod and light intensity conditions

The rule of light on the timing of maturation and spawning in tropical andsubtropical regions is not clear well, because the reproductive cycle in thesesystems is lunar synchronized. In this study, thus, the effects of different lightregimes were investigated on maturational progress of whitespotted rabbitfish,Siganus sutor, the commercial species in Persian Gulf and Oman Sea. Duringprespawning season, 50 adult fish were randomly divided into ten 300-l tanks(n=5). The fish in control tank received indoor light condition and the fish ineach other tanks were exposed to nine different combinations of photoperiod(8L: 16D, 12L: 12D, 16L: 8D) and light intensity (1000, 2000, 3000 lux). After60 days, GSI and HSI values, serum levels of estradiol (E2), testosterone (T),17-α hydroxyprogestrone (OHP), calcium (Ca2+) and gonad histology wereevaluated for females and males.In females the GSI mean values of exposed fish increased in comparison withcontrol except for fish were kept under 8L, 2000 lux (tank 8). These differenceswere significant only for fishes in tank 7 (8L, 3000 lux). In the cases of HSI, theresults were converse, so that, the most of thanks showed significant decreasingthan fishes reared under indoor condition. Morphology and histology study ofOvaries showed three developmental phases including 3, 4 and chiefly 5 thatwere parallel with GSI values. Fortheremore the serum levels of E2 was recordedbetween 0.54-15.8 ng/ml in different fish and their mean values were lower thancontrol in all treatmants (P> 0.05).In males, the similar results were obtained. The GSI and HSI mean values inexperimental regimes compared with control were upper and lower,respectively, except for fishes were reared in tank 1 (16L, 3000 lux). Testeshistology of fishes were reared under different regimes showed signs of stage 5,since no blood vessels observed and thick milt exuded on slight pressure. Themean values of testosterone consentration in fishes were kept in tanks 1 and 6(12L, 1000 lux) were higher and in other ones were lower than control group. Itis also noted that the OHP and Ca2+ had diverse results including increasing anddecreasing mean values than control. So, these factors contrary to E2 and T werenot suitable to evaluation of maturity in both sexes.On the basis of ovarian structure in stage 5, oocyte development pattern in thisspecies was group synchronous. So, increased mean of GSI versus decreasedvalues of HSI, E2 and perhaps Ca2+ were the signs of improved maturation. Butin males, reduced levels of T and similarity of testes morphology in all samplescaused that GSI mean value was the only indicator for differentiating amongtreatments.These findings suggest that alternations were used in light regimes have been thereason of improved maturity in all treatments except fishes reared in tank 8. The١٠٧rule of light intensity on induction of maturity was cleared by comparisionbetween fishes in tanks 4, 5, 6 and control group. Because day length was thesame whereas fishes in tanks 4, 5 and 6 were exposed to increased light intensitycompared with control. This fact verified by results was obtained from fishes intanks 9 and especially 7, since photoperiod in this group was lower than control.So, higher intensity was considered as the reason of alternations. Contrastingwith indoor condition, Induced maturity was also cleared for fishes were kept intanks 1, 2 and 3, where both of light duration and intensity were increased. But,the rule of photoperiod was individually demonstrated when obtained resultswere compared with similar light intensities in other treatments. In conclusion,with comparison among different light regimes it is declared that siganids werekept under light condition of tank 2 including 16h light duration combined with2000 lux intensity showed the best signs of sexual maturation in both sexes.On the basis of this study, setting up the spring light condition duringprespawning season induces maturation in withspotted rabbitfish. Thisimprovement not only by influence of photoperiod or light intensity, separately,but obtained through interaction between them. Thus, determination of thresholdand resistance to light be recommended before exposure, although using properregime during suitable time are necessary to achieve purposes considerably.

PhD

Advisors:A. Matinfar, A. Kamali Counsellors: H. Hosseinzadeh Sahhafi